The viga roof gets a well-deserved boost | Real Estate
A year ago I wrote a column about Rachel Wood, an aptly named New Mexico forest ranger, who is working to bring back local forest products as viable materials for building our homes.
She recently received a grant through the Collaborative Forest Restoration Program to further those efforts. The federal program, passed in 2000, is administered by the United States Department of Agriculture and was the brainchild of former New Mexico Senator Jeff Bingaman.
Her efforts are twofold. One is a chain-of-custody protocol called Good Wood that aims to certify New Mexico’s forests as being as sustainable as those certified by the Forest Stewardship Council, the recognized national standard.
Due to cost and our relatively small market, no New Mexico forests are recognized by the municipality, but all of our forests, whether state, federal, tribal, or private, are sustainably managed.
Wood’s second attempt is to bring back viga span cards. Prior to the changes to the National Codes of 2003, these maps were used by builders, architects and engineers to determine how far a viga of a given diameter could span, at a given distance from each other.
An 8-inch viga, for example, spaced 30 inches apart, could span 16 feet with an assumed load of 40 pounds per square foot. A 12-inch viga could go 30 feet with a similar distance.
Code changes in 2003 said that vigas and rough-cut beams could only be used as structural members if they were assessed and stamped by certified inspectors. In fact, because New Mexico didn’t have one, and still doesn’t, it banned vigas and beams to support roofs. That, as we know from 1,000 years of New Mexico history, was ridiculous, but it went unchallenged.
Historically, vigas used in roofs were relatively small in diameter because it is easier to drag a lean trunk out of the woods than a large one. Also, before flat planks were available at sawmills, thin branches called latillas were laid over the vigas. Then a material, most likely corn cobs, was placed over the latillas and then dirt was piled on top which was tamped and shaped to channel water into channels.
The thick layer of dirt provided nominal insulation and absorbed light rain and snow without dripping too much into the house. Later, with pumice quarried from the Jemez Mountains, that crunchy rice-like material offered similar characteristics but was lighter and more insulating.
Pumice also provided a solid base for layers of tar paper sealed with hot liquid tar topped with gravel to protect tar and paper degradation from our intense sunshine. That method was used until the 1970s, but both the roof and the sand roofs are now illegal and must be removed when re-covering historic buildings.
The bottom line: Those old skinny vigas held up a lot more weight than what would be common practice today, which is spraying lightweight, closed-cell foam directly onto covered planks or latillas.
Wood’s grant provides free engineering to Los Alamos National Laboratory. She has recruited August Mossimon, a laboratory engineer with experience with New Mexico forest products, to lead the development of new span maps. His first calculations indicate that old span graphs were conservative. Instead of 16-foot spans for an 8-inch viga spaced 30 inches apart, his charts show over 20 feet for a Ponderosa trunk and over 23 feet for a Douglas fir log.
With timber prices going through the roof and the pressure on sparse forests to reduce fires and protect our watersheds, bringing back vigas and rafters for roofs and eliminating the redundancy of two roofing systems is a clear win-win situation. State officials should support these efforts.
Kim Shanahan has been a greengrocer in Santa Fe since 1986. Contact him at [email protected]